Process of the production and refining of low-carbon dri (direct reduced iron)

ABSTRACT

A method for the direct reduction of metal oxides with carbon. According to the method, pellets are formed containing a mixture of metal oxides having sequential reduction potentials when heated in the presence of carbon and an amount of carbon sufficient to reduce more easily reduced of the metal oxides yet insufficient to reduce all of the metal oxides. The pellets are heated to a temperature at least sufficient to reduce the more easily reduced metal oxides to produce direct reduction metal while removing sufficient of the carbon in the form of oxides of carbon during the reduction to avoid a subsequent decarburization step in further processing of the direct reduction metal.

RELATED APPLICATIONS

This is a 371 of PCT/CA2012/000387, which claims the benefit of U.S. provisional application 61/457,638, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

This invention relates to metallization of metal oxides in ores and subsequent refining to commercial products such as steels and ferroalloys. This invention particularly relates to carbon levels in solid products of reduction and the subsequent de-carburization of liquid iron and iron alloys in converters.

BACKGROUND

There are two commercially important reactors in current commercial ironmaking: (a) the blast furnace (BF), and (b) the submerged arc furnace (SAF). For kinetic advantages and for permeability of fluid flows, the smelting or melting compartments of these two reactors are packed with metallurgical coke (which is 90% carbon balance ash). In general, the resultant hot metal is very close to carbon saturation (more than 4% from the BF and more than 6% from the SAF) and contains other impurities such as Mn, Si, P, etc. due to carbon reduction of gangue oxides in the raw materials. Carbon and these impurities have to be removed in the subsequent refining processes to produce end products for market.

Different metal oxides may be reduced to their elemental metallic states by reducing agents of different reducing power. A simple example is given here to illustrate the thermodynamic nature of the stability of metal oxides and carbon reduction. When a mixture of metal oxides and carbon (as the reducing agent) is gradually heated up to a high temperature to initiate the reduction reactions, the degree of metallization of each oxide in Direct Reduction Iron (“DRI”) depends on the maximum temperature reached and the amount of carbon available. Based on the thermodynamics of chemical reactions under equilibrium conditions, for example, when a mixture consisting of lead oxide, nickel oxide, iron oxide, manganese oxide, chromium oxide, silica, titanium oxide, alumina is reduced by carbon; these reactions would take place sequentially, following the order as listed of the oxides shown above. After the reduction of all lead and nickel oxides present in the system, the remaining carbon in the mixture will start to reduce iron oxide and so on. At the exhaustion of carbon, all reactions stop. For the mixture of oxides mentioned above, if substantial recovery of chromium is sought, then, the recovery of iron, nickel, lead would be complete and some dissolved Si and Ti would be in the metallic phase at a high enough temperature.

DRI made according to existing commercial “Direct Reduction” processes of ironmaking may be divided into two groups: (1) DRI made in a shaft furnace and rotary kiln which is limited in carbon by chemical reactions, not by supplies, and varies with operating conditions. (2) DRI made in Rotary Hearth Furnaces which has both residual carbon as well as iron oxides with uncertain amounts due to re-oxidation of sponge iron and incomplete reduction.

Ironmaking technology based on the reduction of chemically self-sufficient ore-coal composite pellets in a Paired Straight Hearth (PSH) Furnace can produce DRI with a high degree of metallization and well controlled carbon levels (see for example U.S. Pat. Nos. 6,257,879 and 6,592,648, the entire contents of which are incorporated by reference herein). This is because re-oxidation of sponge iron can be avoided or effectively limited to very low values. Liquid product made by melting DRI of a PSH Furnace has two advantages: (1) selective reduction (SR) to produce low carbon liquid iron through the design of green balls and the use of the PSH Furnace, (2) simpler processing in the refining of low carbon hot metal to end products.

SUMMARY OF THE INVENTION Selective Reduction

The theoretical basis of “Selective Reduction” is based on the thermodynamic nature of the stability of metal oxides and carbon reduction. When a mixture of metal oxides and carbon is gradually heated up to a high temperature to initiate the reduction reactions, the degree of metallization of each oxide in DRI depends on the maximum temperature reached and the amount of carbon available. Based on the thermodynamics of chemical reactions under equilibrium conditions, for example, when a mixture consisting of lead oxide, nickel oxide, iron oxide, manganese oxide, chromium oxide, silica, titania, alumina is reduced by carbon; these reactions would take place sequentially, following the order as listed of oxides shown above. After the reduction of all lead and nickel oxides present in the system, the remaining carbon in the mixture will start to reduce iron oxide and so on. At the exhaustion of carbon, all reactions stop. If, for the mixture of oxides mentioned above, substantial recovery of chromium is desired, then, the recovery of iron, nickel, lead would be complete and there would be some dissolved Si and Ti in the metallic phase at a high enough temperature.

One could apply this concept of “effectively controlled carbon availability” to control the recovery of iron in iron ore below 100% as illustrated in following two examples: One is the smelting of low grade nickel ore in blast furnace, with a typical hot metal containing less than 5% wt Ni and balance mainly iron and carbon. It would be advantageous to increase nickel content through limiting the recovery of iron (but not of nickel) by controlling carbon available in green balls. A higher Ni to Fe ratio in DRI, subsequently in hot metal, will lead to more valuable products. The second example concerns high phosphorous iron ores which are abundant in many parts of world. These have very limited usage in blast furnace ironmaking today because of essentially 100% recovery of P in hot metal and excessive cost of P removal in subsequent steelmaking. In my process ore-coal composite pellets may be designed such that in the melting of DRI there would be a slag of controlled iron oxide level and basicity to keep a substantial portion of the P in the slag.

In summary, my invention is the design of ore-coal composite pellets and the use of a PSH Furnace, so as to take advantage of (1) carbon as the most powerful reducing agent and (2) the introduction of a mechanism to cut-off all reduction reactions at the exhaustion of carbon.

DESCRIPTION OF DRAWINGS

FIG. 1 is a binary phase diagram for the system AL₂O₃—B₂O₃;

FIG. 2 is a binary phase diagram for the system B₂O₃—MgO;

FIG. 3 is a binary phase diagram for the system SiO₂—B₂O₃;

FIG. 4 is a binary phase diagram for the system CaO—B₂O₃;

FIG. 5 is a photomicrograph of direct reduction iron from green balls without flux additions;

FIG. 6 is a photomicrograph of direct reduction iron from green balls with a flux addition of 4 g silica per 100 g of ore; and,

FIG. 7 is a photomicrograph of direct reduction iron from green balls with a flux addition of 5 g of silica and 1.1. g boron oxide in borax.

DESCRIPTION OF PREFERRED EMBODIMENTS De-carburization and Refining of Hot Metal

Currently, liquid metal from ironmaking furnaces is further processed to commercial products often in the same plant. Hot metal from a BF containing 4.5% C and “carbon ferrochrome” from a SAF containing over 6% C are raw materials for making of steels of much less than 1% C and medium carbon ferrochrome of around 2% C, respectively. Therefore, de-carburization is a necessary refining step in the current industrial setting.

Steelmaking

In the case of steelmaking from blast furnace hot metal, there are two necessary and sequential refining steps: (1) de-carburization and removal of dissolved nitrogen and silicon in liquid iron in the Basic Oxygen Furnace (BOF), then, (2) final control of temperature and composition of liquid steel in a Ladle Furnace (LF) before casting. There are two slags produced in traditional flow sheets, one in the BF for the removal of gangue in ore and ash in coal and coke, and the other in a converter for the de-carburization and removal of impurities in hot metal, such as Si, Mn, S and P.

The present invention may be exemplified by the experimental results reported below. Composite pellets (also referred to as “green pellets”) made of typical iron ore and medium or high volatile coal with a C/O ratio (where C stands for total carbon in carbonaceous reductants and O stands for combined oxygen in reducible oxides in ore, both in atomic-grams) in the range of 0.95 to 1.05 may be reduced to DRI of 95% metallization in a laboratory under PSH Furnace operating conditions. Subsequently, the melting of the resultant DRI under a controlled atmosphere, without any flux additions to control S & P, may be carried out in an induction furnace to achieve the separation of liquid metal from slag. The following typical compositions in wt% of liquid metal are given below:

-   For C/O=0.95: C 0.02, Si 0.09, O 0.02, N 0.003 -   For C/O=1.0: C 0.10, Si 0.28, O 0.013, N 0.003 -   For C/O=1.05: C 0.25, Si 0.72, O 0.01, N 0.006

With respect to these four elements, the composition ranges are comparable to liquid steel tapped from a BOF. In current practice, the impurity sulfur (S) is removed partly inside the BF and partly at a de-sulfurizing station between the BF and the BOF. Phosphorus (P) is removed externally first (not in North America because domestic raw materials have low P content), then, in the BOF. In the melting of DRI from the PSH Furnace, the removal of S and P from liquid iron can be effectively done, because the operating conditions can be controlled to have a very high temperature and slags of high basicity (in comparison to the BF) under a neutral or slightly reducing or oxidizing atmosphere. With high phosphorus ore, another de-phosphorization step may be needed to produce high quality steels as in current practice over the world. According to our invention, semi liquid steel from a melter of DRI from a PSH Furnace may be used directly in a LF, i.e., bypassing the BOF for most grades of steel using typical North American raw materials.

Ferrochrome

Ferroalloys are made in a submerged arc furnace (SAF). This is similar to a BF because both are coke based shaft furnaces, although differing in heat source. In the smelting of chromite ore to produce carbon ferrochrome, a higher temperature is required. With electric heating, a SAF can be operated around 1700° C. to satisfy such a requirement.

The most important industrial mineral as a source of chromium is chromite FeO.Cr₂O₃, with some of the Fe replaced with Mg and some of the Cr replaced with Al. The composition of one of many types of spinels may be written as (Mg,Fe)(Cr,Al)₂O₄. Thermodynamically, pure chromium oxide is not much more difficult to reduce than iron oxide. The difficulty in making ferrchrome is due to the fact that chromium oxide in the ore is in the form of spinel minerals. When chromite ore is heated to 1200° C. or higher in the presence of carbon, iron and chromium oxides in spinel (Mg,Fe)(Cr,Al)₂O₄ will be reduced over a period of time to metallic state in the form of very fine particles separated by residual refractory oxides, mainly MgO and Al₂O₃. In industrial practice SAF slag has three major components, MgO, Al₂O₃, and SiO₂ of roughly of comparable amounts. Part of the silica is usually added as flux. The melting ranges in the MgO, Al₂O₃, and SiO₂ ternary system require a very high temperature, around 1700° C., for slag-making and for slag-metal separation and tapping. Ore-coal composite pellets have been used for pre-reduction in a Rotary Hearth Furnace (RHF) to lower power consumption in the SAF.

My invention involves: (1) The making of properly designed composite pellets of chromite ore and coal to produce DRI with a high degree of metallization and controlled and lower residual carbon in PSH type furnaces as explained above. (2) Using a flux added to composite pellets to initiate the making of slag and to promote the growth of metallic particles at a temperature of 1500° C. or lower. (3) Melting hot DRI under a controlled atmosphere in a melter (electric heated furnace or a converter heated by oxy-fuel burner) to produce medium carbon ferrochrome of around 2% Carbon, or lower.

Phase diagrams in the literature show a unique position of boron (B) in the Periodic Table in relation to Mg, Al and Si which indicates that boron oxide would be an effective flux for alumina, magnesia, silica and lime. Four binary phase diagrams of boron oxide (B₂O₃) vs. each of these four oxides are shown in FIGS. 1 through 4 respectively. The effectiveness of boron oxide has been verified by laboratory experiments. Two fluxes in the form of Borax (sodium borate) and Colemanite (calcium borate) were used and found to be effective.

Limited laboratory tests were carried out on a low grade chromite ore with following composition: Cr₂O₃ 42%, Fe₂O₃ 30%, Al₂O₃ 14%, MgO 9%, Silica 1.6% which was mixed with high volatile coal to make green balls (“pellets”) of different composition. These were reduced under similar conditions. A pellet bed with 5 layers of composite pellets, 16-17 mm in diameter, was reduced in an electric muffle furnace in an air atmosphere for 55 minutes (5 minutes at 1200 and 50 minutes at 1500° C.). The effectiveness of boron oxide is clearly shown in FIGS. 5, 6 and 7 which are photomicrographs taken at the same magnification. FIG. 5 shows the results without any flux addition. FIG. 6 illustrates the results using a flux addition of 4 g of silica per 100 g of ore. FIG. 7 illustrates the results from pellets or green balls having a flux addition of 5 g of silica and 1.1 g boron oxide in borax. The bright white phase is metallic and the orange phase is slag.

In order to demonstrate the effectiveness of controlling carbon levels in ferrochrome through the design of green ball compositions, a series of experiments were carried out. Composite pellets of essentially the same ore to coal ratio but different boron oxide (in both sodium and calcium borates) additions were prepared. These green balls were reduced and melted under the same conditions. Three levels of boron oxides, 0.5, 1.1, and 1.5 grams per 100 grams of ore were used. The chemical composition of metal beads obtained by melting DRI was analyzed, the extreme cases (0.5 and 1.5 grams) of boron oxide content are shown in Tables 1 & 2. The results of experiments with boron oxide at the intermediate level were closer to those in Table 2. Furthermore, it has been found that both kinds of borates used are of equal effectiveness for the same amount of boron oxide. Accompanying borates, a second flux such as silica has been found to beneficial when SiO₂ is relatively low in the ore. It is clearly shown that boron oxide in the form of stable compounds such as borates may be added to composite ore-coal pellets to facilitate carbothermic reduction of chromite ore to produce medium carbon ferrochrome directly. It is expected that an optimum composition of green balls for a given set of ore and coal could be found through more experimentation, with respect to lower carbon, or better metal recovery, or both.

TABLE 1 Chemical composition of metal beads from green balls of 0.5 g boron oxide (in borates) per 100 g of ore 1) Metallic Specimen of Fe—Cr Alloy Contents, % Carbon 6.216 Sulphur 0.111 Copper 0.002 Chromium 39.863 Iron 42.568 Phosphorus 0.003 Vanadium 0.435 Silicon 1.3934

TABLE 2 Chemical composition of metal beads from green balls of 1.5 g boron oxide (in borates) per 100 g of ore 1) Sample # 1111 Contents, % Carbon 2.29 Sulphur 0.40 Copper <0.01 Chromium 50.5 Iron 48.70 Phosphorus <0.005 Vanadium 0.56 Silicon 2.0

Modifications of PSH Furnace for Ferrochrome Production

The economic impact as the result of having “PSH Furnace-melter” to replace “BF-BOF” for steelmaking and “SAF-converter” for “medium carbon ferrochrome” manufacturing, with respect to energy consumption, is somewhat different. In addition to using coal instead of coke in both cases, the more significant change in the latter case is that power is replaced by coal as the source of energy in reduction. In a PSH furnace when the reacting pellet bed approaches the discharging end, the rate of energy consumption decreases because reduction is near completion and the rate of generation of gaseous product to protect the top layer of DRI from re-oxidation also diminishes. Priority for control of this end of furnace shifts from the “intensity” of supplying of heat energy to maintain a less oxidizing atmosphere and high temperature to achieve complete metallization and sintering. The last one or two burners near the discharging end may be replaced by a plasma torch for this purpose. The roof height may be lowered in this section to reflect less gas flow above the bed of DRI.

The above description is intended in an illustrative rather than a restrictive sense. Variations may be apparent to persons skilled in the art without departing from the scope of the invention which is defined in the claims set out below. 

1. The direct reduction of metal oxides with carbon comprising the steps of: (i) forming pellets containing a mixture of metal oxides having sequential reduction potentials when heated in the presence of carbon and an amount of carbon sufficient to reduce more easily reduced of said metal oxides yet insufficient to reduce all of said metal oxides (ii) heating said pellets to a temperature at least sufficient to reduce said more easily reduced metal oxides to produce direct reduction metal while removing sufficient of said carbon in the form of oxides of carbon during said reduction to avoid a subsequent decarburization step in further processing said direct reduction metal.
 2. The process of claim 1 wherein said direct reduction metal is iron.
 3. A process for converting the product of the process of claim 2 into steel comprising the further steps of: (iii) melting said DRI in a melter in the presence of high basicity slag to control sulphur and phosphorous levels and to convert said iron into steel, thereby bypassing the use of a BOF for carbon reduction and allowing the use of raw materials having high sulphur and phosphorus contents not suitable for BR and SAF processes.
 4. A process for producing ferrochrome from chromite ore comprising the steps of: (i) forming from pellets a mixture of said chromite ore, coal and at least one flux, said coal being present in an amount sufficient to substantially reduce iron and chromium oxides in said ore without leaving more than a predetermined amount of residual carbon; said flux reacting with refractory metal oxides to cause partial melting of said refractory oxides; (ii) heating said pellets from step (i) to cause said flux to initiate melting of said refractory oxides to form slag and to reduce said iron and chromium oxides to form an iron and chromium mixture containing residual carbon said flux promoting the reduction and the growth in size of metallic and slag phases to assist in controlling of the amount of residual carbon (dissolved in elemental form) in direct reduction metal resulting from said reduction; and (iii) separating said iron/chromium/residual carbon mixture from said slag.
 5. The process of claim 4 wherein the at least one flux includes borates.
 6. The process of claim 5 wherein said iron and chromium mixture and any residual carbon therein is melted in a melter without decarburization to produce a medium carbon ferrochrome alloy.
 7. The use of a fluxing agent in a process for thermal reduction of one or more selected metal oxides with carbon wherein the selected metal oxides are present in interstices of a crystal structure having at least one higher melting metal oxide, wherein said fluxing agent depresses the melting point of said one or more higher melting metal oxides to promote melting of said one or more higher melting metal oxides thereby freeing any reduction products of said selected metal oxides from said crystal structure, and promoting the reduction and the growth in size of metallic and slag phases to assist in controlling the amount of residual carbon dissolved in said reduction products. 